The minimum OSD config tool doesn't allow callsigns shorter than 6 characters.
Some of us have Amateur (HAM) callsigns that are significantly shorter than 6 characters (I have a four character call sign).
I was able to work around the limitation by entering two spaces in front of my call sign.
I hope the tool can be updated to fix this limitation.
Perhaps this will help others that have short callsigns until the ArduCAM OSD Config tool can be updated (I was using version 22.214.171.124).
I just downloaded the file "CT for MiniumOSD Extra 2.2", which shows version "126.96.36.199" and it allows a shorter callsign. So it appears that this was fixed in later versions.